any structural anomaly of the embryonic connective tissue made up of loosely aggregated mesenchymal cells, supported by interlaminar jelly, that gives rise to the developing metanephros; interactions between the ureteric bud and the metanephric mesenchyme leads to the initiation of outgrowth and repetitive branching of the UB that ultimately generates the definitive renal collecting system and induces formation of renal vesicles from the mesenchyme tissue